1. Field of the Invention
This invention relates to a deflection compensating roller which comprises a rotating roller casing, a rotationally fixed yoke axially extending through the roller casing, and at least one hydrostatic support source arranged between the yoke and the roller casing, which on its end facing the roller casing is provided with at least one oil pocket that communicates with the pressure compartment of the support source via at least one first restricted flow zone.
2. Discussion of Background Information
In such deflection compensating rollers use is made of support sources which are pressurized by a supply line with oil pressure. This oil pressure presses the support source against the rotating roller casing. As the piston area of the support source is smaller than the pocket area facing the roller casing, a lower pocket oil pressure results. The pressure difference between the piston pressure and the pocket pressure defines the volume flow which flows via the capillaries that are connected between the pocket area and the piston area. Hence the volume flow which results at a support source depends on the piston pressure.
For individual profile correction of the product web running through the nip the support sources are pressurized with an oil pressure. The level of the oil pressures is controlled via online profile thickness measurement of the product web. However, depending on the profile corrections required this can result in large oil pressure differences at the support sources (e.g. 3.5 to 90 bar from support source to support source). This leads, as previously mentioned, to volume flow differences at the support sources. Friction arises between the rotating roller casing and the support sources due to the oil shear, which is dependent on the casing speed and the oil gap width, which in turn is dependent on the volume flow, the oil temperature and the pocket pressure.
Owing to the large pressure differences the result therefore is a level of friction energy which differs from one support source to another and results in temperature differences on the roller casing. These temperature differences have an effect in turn on the shape of the roller casing and hence a feedback effect on the formative distributed load profile of the deflection compensating roller.
Pressure relief at a support source results in the smaller volume flow, hence a higher temperature results at this support source in spite of a smaller friction energy in quantity terms than with high pressures. However, a higher temperature leads to an expansion of the roller casing, which results in a distributed load increase in the nip. The temperature development in question makes itself felt inverse to the desired pressure relief and is therefore undesirable. In some cases it can even lead to an instability of the control response.
Up to now it was generally customary to limit the temperature development at the support sources by a separate cooling current which is directed into the interior space of the roller. For this purpose a volume flow of low temperature was distributed in the inside space of the roller by means of nozzles, its quantity being controlled by way of the return run temperature of the roller. As the result of this distribution, each support source is fed with the same quantity of cooling oil. Owing to the previously described volume flow differences, different temperatures result at the support sources in spite of the supplied quantity of cooling oil. The mixed temperature resulting in the inside space of the roller corresponds roughly to the local return run temperature.
The resulting local return run temperatures display an increasing temperature difference between a high-loaded support source and a low-loaded support source as the cooling oil flows grow smaller. This temperature difference has a decisive effect on the shape of the rotating roller casing.
From DE 101 36 270 A there is already known a controlled deflection roller on which the quantity of fluid supplied locally to a respective roller zone by means of a temperature controlling device is variable as a function of the piston pressure of the support element in question or the support element group in question.